John Steinbeck takes the title of this novel from the poem "To a Mouse [on turning her up in her nest with the plough]," written by Scottish poet Robert Burns in 1785.
In the poem, the speaker has accidentally turned up a mouse's nest with his plough. He pauses for a little rumination about how men and animals might seem different, but in the end they're all mortal. No matter how different "thinking men" and "unthinking animals" seem, everybody suffers and dies in the end:
But, Mousie, thou art no thy lane [alone]
In proving foresight may be vain:
The best-laid schemes o' mice an' men
Gang aft agley, [often go awry]
An' lea'e us nought [leave us nothing] but grief an' pain,
For promised joy.
But there is one difference. Mice and men might both die, but only the men are aware of it. In the last verse of the poem, Burns' speaker says that the mouse is "blest":
Still thou art blest, compar'd wi' me!
The present only toucheth thee:
But, och! I backward cast my e'e [eye]
On prospects drear! [dreary]
An' forward, tho' I canna [cannot] see
I guess an' fear!
In other words, the mouse can't think about the past or the future. Does this remind you of anyone? Us, too. It seems like Steinbeck is thinking of Lennie as the mouse, and George as the man who turns up its nest: life messes them both up, but at least Lennie doesn't have to remember any of it. Whatever happens to Lennie is over. He doesn't regret anything and he doesn't anticipate anything—not even his death.
But not George. George will have to live with what he's done for the rest of his life.